1. Field of the Invention
This invention relates to a new, glass fiber reinforced, high strength, high rigidity polyamide composition having a superior surface appearance and also to a process for manufacturing such a polyamide composition.
2. Prior Art
Crystalline aliphatic polyamide resins are widely used in the industries as mechanical parts, electric parts, and automobile parts, because it has good mechanical properties. Especially when it is compounded with glass fibers, the strength, rigidity, heat resistance and impact strength are greatly improved.
However, the performance of the glass fiber reinforced aliphatic polyamide of the prior art, namely glass fiber reinforced nylon 6 or nylon 66, have hardly developed in these fields. The result for this is that, the limit of the tensile strength specified in ASTM D638 of the glass fiber reinforced aliphatic polyamide of the prior art lies between 20 to 22 kg/mm.sup.2 and the flexural modulus is 1200 kg/mm.sup.2, and these values decrease to 50 to 60% at the real use condition of 23.degree. C., 50% RH since the amide group of nylon absorbs a large amount of water to make the molecular chain soft. Thus, the materials made by the prior art are hardly acceptable to replace metal materials which are, for example, aluminum die casting alloy (JIS ADC 10) that has a tensile strength of 28.8 kg/mm.sup.2, or zinc die casting alloys (JIS ZDC1, ZDC2) that have a tensile strength of 33 and 29 kg/mm.sup.2, respectively. Therefore, many attempts have been made to improve the property of the prior art materials.
A polyamide resin developed for such an object includes a polyamide made by introducing aromatic hydrocarbons into the molecular chain. Such a material is so called partially aromatic polyamide comprising a component which has an aromatic ring in any one of the starting materials of conventional polyamides, diamine and dibasic acid. An example of such a polyamide is polyamide MXD-6 made by polycondensation of metaxylylene diamine and adiphic acid. The introduction of aromatic hydrocarbon into the molecular chain increases the rigidity or stiffness of the material. In addition, since the introduction of aromatic hydrocarbon reduces water absorption, the decrease of physical properties by the absorption of water is improved. However, the impact resistance is decreased since the molecular chain becomes brittle.
From the viewpoint of processing, the stiffness of the molecular chain reduces the melt flow of the material, and also retards the rate of crystallization. Therefore, it is generally required that the material must be cooled slowly to obtain higher crystalinity. However, since this molding condition for this object requires higher mold temperature as 130.degree. C. which prolongs the molding cycle, to gradually cool down the material is extremely unfavorable for the processing. In addition, the resulting incompleteness of the crystallization makes the scattering of physical properties and the dimensions of the molded parts larger.
Since the prior art cannot produce a polyamide material which is balanced in processability, impact resistance and toughness and economical in cost, the improvement is eagerly required by the industries.
Another drawback of the prior art is the limitation of glass fiber loading. An amount of the glass fiber which is contained in the glass fiber reinforced polyamide composition is usually 45 to 70 weight parts for 100 weight parts of resin for most commercial use, and the grass fiber content is limited to 100 weight parts for 100 weight parts of resin for the uppermost filled resin.
In the higher loading of glass fiber, fiber conglomeration and poor dispersion within the matrix resin cause the transfer of force to become imperfect, and the mechanical strength and rigidity of the resin is decreased. In addition, many voids which is generated at the interface of fibers and resin cause the decrease of the strength of the material notwithstanding that the glass fiber content is increased.
Furthermore, glass fibers at the surface of the molded parts decrease the gloss and the flatness of the surface, and lower the commercial value of the molded parts; therefore, higher glass fiber loading is hardly applied to real use, and when the molded parts are painted, the rough surface absorbs the paint and a uniform surface is unattainable.
Since the mechanical properties such as tensile strength and flexural modulus are proportional to the volume ratio of the reinforcement, the composites made by the prior art can hardly replace metal parts, and the glass fiber loading level is limited to 15 to 30% by volume. As a result, the mechanical properties of the glass fiber reinforced polyamide composite remain in lower levels.